Tracheobronchial epithelium of the sheep: III. Carbohydrate histochemical and cytochemical characterization of secretory epithelial cells

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Abstract
We examined histochemically (light microscopy‐LM) and cytochemically (electron microscopy‐EM) the secretory epithelial cells in the tracheobronchial mucosa of sheep. Six morphologically distinct, granule‐containing cells have been described, on the basis of their morphology and airway distribution: four mucous (M1–M4), serous (SC), and Clara (CC). Stereological and morphometric data indicated that M3, M4, SC, and CC were distinctly different from each other and from M1 and M2 cells. Mucous cells M1 and M2 differed in granule morphology. Samples of tracheas, sixth‐generation bronchi, distal bronchi, and terminal bronchioles of 18 adult sheep were examined. At the LM level, methacrylate sections were reacted with an alcian blue (pH 2.5), periodic acid Schiff (PAS) sequence to differentiate neutral from acidic glycoconjugates (GC), and a high‐iron diamine (HID), alcian blue sequence to differentiate sulfated from nonsulfated (sialated) GC. At the EM level the periodic acid–thiocarbohydrazide localized hexose‐rich, neutral GC. Dialyzed iron (DI) and high‐iron diamine localized carboxylated and sulfated GC, respectively. Granules of all but Clara cells were PAS‐positive. All mucous cells contained acidic groups, but only M1 and M4 cells had LM‐detectable sulfated GC. At the ultrastructural level, minimal but discernible HID and LID reaction product was observed on granule profiles of M2, M3, and SC, indicating acidic and sulfated GC not detected at the LM level. Histochemically, the sheep tracheobronchial epithelium was more similar to that of humans than some other examined mammalian species.